About this indicator
Excess nitrogen and phosphorus loading impacts not only local waters, but also downstream waterbodies and coastal systems including the Chesapeake Bay, the Great Lakes, the Gulf of Mexico, and Puget Sound. The data in this indicator table are based on output from the United States Geological Survey’s (USGS) Spatially Referenced Regression On Watershed Attributes (SPARROW) models. SPARROW is a watershed modeling tool which, based on a mass-balance approach, estimates the excess amounts (i.e., amounts beyond assimilative capacity) of nitrogen and phosphorus exported from watersheds and delivered to downstream waterbodies. The models relate in-stream water-quality measurements of nutrients taken at a network of monitoring stations to spatially referenced attributes of the corresponding watersheds, such as nutrient sources and environmental factors that affect rates of delivery to streams, as well as in-stream processing of nutrients.

Estimated state-wide total nitrogen (TN) and total phosphorus (TP) loads and yields, as predicted by the 2002 regional USGS SPARROW models.

State

Net incremental TN load (1000 kg/yr)

Average TN yield (kg/yr/km2)

Net incremental TP load (1000 kg/yr)

Average TP yield (kg/yr/km2)

Alabama

68,506

512

6,381

48

Alaska*

No data

No data

No data

No data

Arizona*

No data

No data

No data

No data

Arkansas

97,016

704

15,216

110

California*

Insufficient data

Insufficient data

Insufficient data

Insufficient data

Colorado**

13,311

89

2,043

14

Connecticut

13,242

1,028

1,320

102

Delaware

6,738

1,303

477

93

Florida**

37,286

359

7,465

72

Georgia

60,007

396

6,168

41

Hawaii*

No data

No data

No data

No data

Idaho

43,989

212

5,171

25

Illinois

255,337

1,750

15,043

103

Indiana

212,421

2,268

6,742

72

Iowa

290,402

1,993

15,558

107

Kansas

97,610

458

14,288

67

Kentucky

303,697

2,902

9,266

89

Louisiana

110,854

917

23,811

197

Maine

24,604

293

1,870

22

Maryland

29,078

1,095

3,151

119

Massachusetts

17,548

837

1,279

61

Michigan

101,740

685

4,876

33

Minnesota

162,845

751

10,177

47

Mississippi

78,267

634

13,078

106

Missouri

357,138

1,978

25,150

139

Montana

23,767

62

5,959

16

Nebraska

73,539

367

9,616

48

Nevada*

Insufficient data

Insufficient data

Insufficient data

Insufficient data

New Hampshire

9,529

396

536

22

New Jersey

34,782

1,782

4,597

236

New Mexico*

Insufficient data

Insufficient data

Insufficient data

Insufficient data

New York

116,095

927

12,285

98

North Carolina

59,194

465

6,493

51

North Dakota

44,279

245

3,267

18

Ohio

231,831

2,171

8,788

82

Oklahoma

76,891

425

9,976

55

Oregon**

75,037

323

8,641

37

Pennsylvania

132,627

1,131

7,880

67

Rhode Island

3,879

1,365

318,106

112

South Carolina

28,173

355

3,049

38

South Dakota

34,007

170

5,246

26

Tennessee

86,030

788

16,241

149

Texas**

202,028

363

25,269

45

Utah*

Insufficient data

Insufficient data

Insufficient data

Insufficient data

Vermont

11,209

450

644

26

Virginia

61,989

596

5,498

53

Washington

70,493

407

8,358

48

West Virginia

32,139

512

1,977

32

Wisconsin

109,244

754

6,681

46

Wyoming**

7,969

41

2,469

13

Note - These data reflect the excess amounts ('loads') of TN and TP generated beyond the assimilative capacity of each states' watersheds. Net incremental TN or TP load is the sum of the area-weighted++ incremental TN or TP loads from all catchments (drainage basins) within a state that are covered by one or more regional SPARROW models. Also shown is the average yields for each state, which is the net incremental TN or TP load within a state divided by the area of the state with SPARROW model coverage.

* Denotes states for which 2002 SPARROW model data were unavailable for most or all of the state area.

** % of state area covered by respective 2002 SPARROW model(s): Colorado - 56%; Florida - 75%; Oregon - 93%; Texas - 82%; Wyoming - 78%. Values shown only reflect area covered. All other states are covered within the range of 97-100% of state area.

++Where only a portion of a catchment was within state boundaries, the contribution from that catchment was calculated as the incremental load from the catchment scaled by the portion of the catchment within the state's boundaries.

Data source information
State-level estimates of TN and TP loads and yields were made as follows:

USGS 2002 regional SPARROW model outputs for TN and TP incremental yields (kg/yr/km2) were obtained as CSV files from the USGS SPARROW Decision Support System (DSS) for each of the six geographical regions (referred to as ‘Major River Basins’) for which USGS has generated 2002 estimates. Data files were merged in Microsoft ACCESS and converted to DBF (DataBase Files) format for use with GIS (Geographical Information System) software.

Geospatial files (i.e., GIS polygons or shapefiles) representing the hydrologic networks used in the 2002 SPARROW models were obtained from USGS. The New England and Mid-Atlantic (Major River Basin 1 or MRB1) model was based on NHD (National Hydrography Dataset) Plus catchments, whereas all other 2002 models were based on MRB_E2RF1 (Major River Basin_Enhanced River Reach File) catchments.

The hydrologic network data and associated DSS output from all of the models were joined in ESRI’s ArcGIS v.10 to generate two seamless grids of incremental yield estimates (kg/yr/km2): one for TN and one for TP. The DSS output for each catchment was linked to the hydrologic network data using the “ID” attribute field in the DSS output and the appropriate unique identifier for the hydrologic network file (“GRID_CODE” attribute field for NHDPlus catchments, and “Value” attribute field for MRB_E2RF1).

Note: Because NHDPlus and MRB_E2RF1 catchments are of significantly different scales, the model extents/boundaries between the model using NHDPlus catchments (New England and Mid-Atlantic model or “MRB1”) and adjacent models using MRB_E2RF1 catchments (Southeastern US or “MRB2” and Upper Midwest or “MRB3”) have small overlaps and gaps. Overlapping areas were assigned the yield estimates from the MRB2 or MRB3 model by using the Erase tool of Xtools Pro (an add-on toolbar for ArcGIS) to remove the yield estimates from the MRB1 model. No estimates of incremental yields were assigned to the gaps. (However, note that these gaps are accounted for in the percent of state area covered by the respective model(s), which was used to determine incremental yields.)

Xtools Pro was used to calculate the area of all model catchments and the ArcGIS Intersect tool was used—with the state boundaries file from the 2010 U.S. Census—to limit the corresponding TN or TP incremental yields to model output data from catchment areas within state boundaries. This was done so that incremental loads for catchments that cover more than one state were apportioned using an area-weighted approach. All data were then exported into Microsoft Excel for post-GIS processing.

For each catchment within state boundaries, incremental load (kg/yr) was determined by multiplying the incremental yield (kg/yr/km2) by the area of the catchment (km2) within state boundaries (determined in previous step). Net incremental load is the sum of all incremental loads within state boundaries.

Average yield for all catchment areas within the boundaries of each state was determined by dividing the net incremental load of TN or TP within state boundaries by the total state area covered by one or more of the regional models.

Note - Any reference to trade names or commercial products is for informational purposes only and does not constitute any endorsement or recommendation for use. US EPA and its employees do not endorse any commercial products, services or enterprises.

What to consider when using these data
With the exception of Florida and some western states, 2002 SPARROW model coverage of states is high, in the range of 97% - 100% of state area. In that coverage range, states without 100% coverage are generally coastal states and areas not covered by DSS output are generally coastal regions. Although 75% of Florida is covered in the 2002 South Atlantic Gulf and Tennessee (region 2) SPARROW model, significant portions of central Florida and all of south Florida are excluded. For Texas, Colorado and Wyoming, a significant and likely representative portion of the state (82%, 56%, and 78%, respectively) is covered in at least one of the 2002 regional SPARROW models. Because of the lack of complete or near complete coverage of all states, loads and yields of TN and TP were calculated on the basis of the area of each state currently covered in the 2002 models. For states with a very low or zero coverage by the 2002 regional models, estimates of TN and TP loads and yields were not made (indicated by ‘No data’ or ‘Insufficient data’ in the data table). Users can see the six regions/major river basins of the U.S. covered on USGS’ website on Regional SPARROW Model Assessments of Streams and Rivers. Model output for region 6 (Rio Grande, Colorado, and Great Basin) and region 8 (California) are currently not available.

Like all models, the SPARROW models have uncertainty associated with them. To minimize uncertainty (or the unexplained differences between measured and predicted values, as measured by each model’s coefficient of determination [R2 value] and root mean squared error), USGS developed their 2002 regional SPARROW models using water quality monitoring sites for model calibration, and geospatial datasets on source contributions and delivery factors such as basin characteristics and water flow. In all cases, the load R2 values for both TN and TP models are ≥ 0.84, and 8 of the 12 models have load R2 values that are ≥ 0.90 (Preston et al. 2011). Additionally, on the DSS website, users can view, on a regional model basis, the standard error of the estimated incremental TN and TP loads for each catchment or reach. Due to complications associated with translating uncertainty at the level of stream reaches to an estimate of uncertainty for the overall load and yield per state, state-wide values of uncertainty have not been calculated.